Easiest process for me to apply textures, created externally, to my models
You have to UV unwrap your model, lay it out the way it should be (and then don't change it). Send that UV layout to the texture artist. 

That is the only way you can have textures custom built for any 3D object.
The person painting the texture, you or somebody else makes no difference, has to know where to put and how to shape the texture. (How could you paint a room if you did not know what the room looked like?)



You can export the UVs from the UV/Image Editor's header by clicking on UVs > Export UV Layout.
Blender's file browser will open up letting you choose the name and location for the new image. There are some important settings on the left side.



Make sure the export size is the same size that your target UV map will be.
Set the Fill Opacity to something higher then the default of .25 (the default is too transparent to be useful). I like 1 as it is completely opaque which gives a good base layer to work off of.